processes Article
Effect of Particle Size on Carbon Nanotube Aggregates Behavior in Dilute Phase of a Fluidized Bed Sung Won Kim
ID
School of Chemical and Material Engineering, Korea National University of Transportation, Chungju-si, Chungbuk 27469, Korea;
[email protected]; Tel.: +82-43-841-5228
Received: 30 June 2018; Accepted: 6 August 2018; Published: 8 August 2018
Abstract: Fluidized bed reactors have been increasingly applied for mass production of Carbon Nanotube (CNT) using catalytic chemical vapor deposition technology. Effect of particle size (dp = 131 µm and 220 µm) on fluidization characteristics and aggregation behavior of the CNT particles have been determined in a fluidized bed for its design and scale-up. The CNT aggregation properties such as size and shape were measured in the dilute phase of a fluidized bed (0.15 m-ID × 2.6 m high) by the laser sheet technique for the visualization. Two CNT particle beds showed different tendency in variations of the aggregates factors with gas velocity due to differences in factors contributing to the aggregate formation. The CNT particles with a larger mean size presented as relatively larger in the aggregate size than the smaller CNT particles at given gas velocities. The aggregates from the large CNT particles showed a sharp increase in the aspect ratio and rapid decrease in the roundness and the solidity with gas velocity. A possible mechanism of aggregates formation was proposed based on the variations of aggregates properties with gas velocity. The obtained Heywood diameters of aggregates have been firstly correlated with the experimental parameter. Keywords: carbon nanotube; fluidized bed; aggregates; particle size; laser sheet technique
1. Introduction Carbon Nanotube (CNT) is a very promising material with a great potential for a wide range of applications, including electronic materials and energy fields [1]. In recent years, fluidized bed reactors have been increasingly applied for mass production in the CNT synthesis using catalytic chemical vapor deposition (CCVD) technology [2,3]. Since the fluidized bed reactor has a high heat and mass transfer efficiency, it is advantageous for the synthesis of CNTs with uniform quality, and it is evaluated as a method capable of continuous production and mass production of the CNTs [1,3]. The strong anisotropy of the nanotubes on particle can affect fluidization behavior of the particles in the fluidized bed reactor because the CNT particles fall under the Geldart C classification. It is difficult to handle the CNT particles in a gas-solid fluidized bed reactor because the cohesive force between the particles is stronger than the hydrodynamic force by fluid medium. The aggregation of primary particles in the fluidized bed is dependent on the inter-particle interactions, which are a function of the primary particle size [4,5]. Since the hydrodynamic behaviors of the CNT particles in fluidized beds are governed by the interaction of individual particles, an accurate understanding of these micro-scale interactions may assist in the hydrodynamic modeling, design and scale-up of the process. However, the aggregates by the interactions have the inherently dynamic and chaotic properties in meso-scale flow structure of gas-solid flow in the fluidized bed. The measurement and characterization of the aggregates are complicated due to the dynamic properties.
Processes 2018, 6, 121; doi:10.3390/pr6080121
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Intrusive and non-intrusive methods have been used to measure the aggregate properties in the gas-solid fluidized beds [6]. The intrusive methods such as capacitance probing, optical fiber Intrusive and non-intrusive methods have been used to measure the aggregate properties in the probing and momentum probing is advantageous in simplicity of setup and cost. However, they gas-solid fluidized beds [6]. The intrusive methods such as capacitance probing, optical fiber probing disturb the local flow of particles. The non-intrusive methods such as laser sheet technique and and momentum probing is advantageous in simplicity of setup and cost. However, they disturb the tomography technique measure the whole image of the particles flow in a wide range with local flow of particles. The non-intrusive methods such as laser sheet technique and tomography non-invasive and non-contact advantages [7]. The imaging based on laser sheet technique has been technique measure the whole image of the particles flow in a wide range with non-invasive and used to measure aggregation phenomena at different flow regime in fluidized beds with advantages non-contact advantages [7]. The imaging based on laser sheet technique has been used to measure of easy installation and in-situ measurement [6,8–10], which is originally popular method in the field aggregation phenomena at different flow regime in fluidized beds with advantages of easy installation of single-phase flow and gas-liquid flow analysis [11]. Recently, a few studies [6,8] have used the and in-situ measurement [6,8–10], which is originally popular method in the field of single-phase laser-based image analysis method to measure sizes of the multi-walled CNT aggregates. Jeong and flow and gas-liquid flow analysis [11]. Recently, a few studies [6,8] have used the laser-based image Lee [6] measured the aggregate size just above the bed surface for inferring particles behavior in the analysis method to measure sizes of the multi-walled CNT aggregates. Jeong and Lee [6] measured dense bed, not in the dilute phase of fluidized bed. Kim [8] has focused predominantly on the the aggregate size just above the bed surface for inferring particles behavior in the dense bed, not in visualization of the CNT particles behavior in the freeboard region within a narrow range of gas the dilute phase of fluidized bed. Kim [8] has focused predominantly on the visualization of the CNT velocity with one kind of the CNT particles. However, the aggregation behavior of nanoparticles is particles behavior in the freeboard region within a narrow range of gas velocity with one kind of the strongly affected by the initial particle size of the bed [4,5,9]. The study on primary particle size CNT particles. However, the aggregation behavior of nanoparticles is strongly affected by the initial dependence of the CNTs aggregation behavior is still lacking to the best of our knowledge, even particle size of the bed [4,5,9]. The study on primary particle size dependence of the CNTs aggregation though a few reports have been made on nano-powders [4,5]. Therefore, a further study on the behavior is still lacking to the best of our knowledge, even though a few reports have been made on fluidization characteristics of CNT aggregates in the dilute phase of the fluidized bed is required for nano-powders [4,5]. Therefore, a further study on the fluidization characteristics of CNT aggregates in obtaining the design factors of freeboard part in the CNT reactor such as the heat transfer modeling the dilute phase of the fluidized bed is required for obtaining the design factors of freeboard part in [12] and the cyclone design [13]. the CNT reactor such as the heat transfer modeling [12] and the cyclone design [13]. The objective of this study is to determine the effect of CNT particle size on fluidization The objective of this study is to determine the effect of CNT particle size on fluidization characteristics and aggregates behavior of the CNT particles in a fluidized bed. The CNT aggregates characteristics and aggregates behavior of the CNT particles in a fluidized bed. The CNT aggregates properties were measured based on the imaging analysis using the laser sheet technique to analyze properties were measured based on the imaging analysis using the laser sheet technique to analyze the the dynamic behavior of the CNTs of different size in the dilute phase of the reactor. dynamic behavior of the CNTs of different size in the dilute phase of the reactor. 2. 2. Materials Materials and and Methods Methods 2.1. Materials and Experimental Apparatus In order to investigate the fluidization behavior of CNTs, two kinds of CNT particles were used as the bed materials. materials. They have have the the same same particle particledensity density(ρ (ρpp == 162 162 kg/m kg/m33))but butdifferent different mean mean particle diameters (CNT I = 131 µm, μm, CNT II == 220 µm). μm). The minimum minimum fluidization fluidization velocities velocities of of the CNT m/s for m/s for particles are 0.0026 m/s for CNT CNT I and 0.0037 m/s for CNT CNT II, II, respectively. respectively. The The Raman spectrum of the CNT used in this study is shown in Figure 1a, featured in a weak radial radial breathing breathing modes modes (RBMs) (RBMs) −1 − 1 −1 , near 200 cm , the higher frequency D (disordered), G (graphite) in the range of 1200–1700 cm , and 200 cm , the higher frequency D (disordered), G (graphite) in the range of 1200–1700−1cm −1 − 1 G’ around 27002700 cm cm . The. The particle size size distribution andand SEMSEM images of CNT I and CNT II andmodes G’ modes around particle distribution images of CNT I and CNT particles areare shown in Figure 1b–d. As As shown in the figure, the CNT particles havehave various sizes.sizes. The II particles shown in Figure 1b–d. shown in the figure, the CNT particles various nanotubes are entangled withwith eacheach other on the of the The nanotubes are entangled other on surface the surface of particles. the particles.
Figure 1. Cont.
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Figure 1. Characteristics of multi-walled Carbon Nanotube (CNT) particles in this study: (a) Raman Figure 1. Characteristics of multi-walled Carbon Nanotube (CNT) particles in this study: (a) Raman spectrum analysis; (b) comparison of particle size distribution of CNT I and CNT II; (c) SEM images Figure 1.analysis; Characteristics of multi-walled Carbon Nanotube (CNT) particles thisII;study: (a) Raman spectrum (b) comparison of particle size distribution of CNT I and in CNT (c) SEM images of of CNT I; (d) SEM images of CNT II. spectrum analysis; (b) comparison of particle size distribution of CNT I and CNT II; (c) SEM images CNT I; (d) SEM images of CNT II. of CNT I; (d) SEM images of CNT II.
Experiments were carried out in a fluidized bed unit made of transparent Plexiglas column as Experiments were carried out fluidized bed unit m-ID made ×of2.0 transparent Plexiglas column as shown in Figure 2. It consisted of in a amain column m high)Plexiglas with a cyclone. Experiments were carried out in a fluidized bed(0.15 unit made of transparent column asThe shown in Figure 2. It consisted of a main column (0.15 m-ID × 2.0 m high) with a cyclone. The column column expanded 0.30 m toofreduce of particles. type distributor was used shownwas in Figure 2. Ittoconsisted a mainelutriation column (0.15 m-ID × A 2.0tuyere m high) with a cyclone. The was expanded to 0.30 m to reduce elutriation of particles. A tuyere type distributor was used to to column inject air for fluidization. Fluidizing air was introduced into the column using a mass flow meter. was expanded to 0.30 m to reduce elutriation of particles. A tuyere type distributor was used inject air for fluidization. Fluidizing air was introduced into the column using a mass flow meter. Pressure were located axially at theair wall the column to the measure pressure for obtaining to injecttaps air for fluidization. Fluidizing wasofintroduced into column using a drops mass flow meter. Pressure taps axially at wall of the the column measure pressure drops obtaining thePressure information of located solid holdup. materials of 1.0 kg was loaded pressure and the static bed height was tapswere were located axiallyBed at the the wall of column totomeasure drops forfor obtaining the information of solid holdup. Bed materials of 1.0 kg was loaded and the static bed height was 0.60 m. The gas velocity was varied 0.09–0.20 m/s. the information of solid holdup. Bed materials of 1.0 kg was loaded and the static bed height was 0.60 m.m. The m/s. 0.60 Thegas gasvelocity velocitywas wasvaried varied 0.09–0.20 0.09–0.20 m/s.
Figure and schematicdiagram diagram of laserlight light sheet method. Figure2.2.Experimental Experimental apparatus apparatus and sheet method. Figure 2. Experimental apparatus and schematic schematic diagramofoflaser laser light sheet method.
high-speedcamera camera(RX100M4, (RX100M4, Sony, Tokyo, thethe column at the A Ahigh-speed Tokyo, Japan) Japan)was wasinstalled installedoutside outside column at the heightofof1.5 1.5mmfrom fromthe thedistributor distributor location to inside thethe height to visualize visualizethe theCNT CNTaggregates aggregatesbehavior behavior inside
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A high-speed camera (RX100M4, Sony, Tokyo, Japan) was installed outside the column at the Processes 2018, 6, x FOR PEER REVIEW 4 of 12 height of 1.5 m from the distributor location to visualize the CNT aggregates behavior inside the reactor and determine their properties such as size and shape. The light emitted from a He-Ne laser reactor and determine their properties such as size and shape. The light emitted from a He-Ne laser (Model 1145, Lumentum, Milpitas, CA, USA) passed through a grass rod lens and column wall, (Model 1145, Lumentum, Milpitas, CA, USA) passed through a grass rod lens and column wall, and and illuminated the inside of the column. Resolution, sampling rate and exposure time of the camera illuminated the inside of the column. Resolution, sampling rate and exposure time of the camera were 350 dpi, 480 frames/s and 1000 µs, respectively. were 350 dpi, 480 frames/s and 1000 μs, respectively. 2.2. Image Analysis Method 2.2. Image Analysis Method To minimize the uncertainties from dynamic flow state in the fluidized bed, about 30 images were To minimize the uncertainties from dynamic flow state in the fluidized bed, about 30 images obtained with a time interval of approximately 3 minutes after ensuring the steady state operation by were obtained with a time interval of approximately 3 minutes after ensuring the steady state visual observation and pressure measurement. The Image J [14], developed by National Institute of operation by visual observation and pressure measurement. The Image J [14], developed by Standards and Technology US, was applied to process the image obtained as in Figure 3. The original National Institute of Standards and Technology US, was applied to process the image obtained as in image (Figure 3a) was first converted to Figure 3b by using the contrast function to enhance the directly Figure 3. The original image (Figure 3a) was first converted to Figure 3b by using the contrast reflected light from the CNT particles on the light sheet and to eliminate the interference of the ambient function to enhance the directly reflected light from the CNT particles on the light sheet and to reflected light such as the reflection from the inner wall of the reactor and the reflection from the eliminate the interference of the ambient reflected light such as the reflection from the inner wall of particles outside of the light sheet. The contrasted image of Figure 3b was converted to threshold the reactor and the reflection from the particles outside of the light sheet. The contrasted image of ones, as3b seen in converted Figure 3c, to to threshold identify the aggregates after removing background light after noise, Figure was ones, as seen in Figure 3c, to some identify the aggregates which is called thresholding [9]. After the thresholding, the function of ‘Analyze Particle’ in the Image removing some background light noise, which is called thresholding [9]. After the thresholding, the J function providesofmeasured ofin the aggregates properties such asvalues shape of factors, size and area for the ‘Analyze values Particle’ the Image J provides measured the aggregates properties identified particles as in Figure 3d. for the identified particles as in Figure 3d. such as shape factors, size and area
Figure 3. Image processing of CNT aggregates (a) Original image; (b) Removal of noise; (c) Figure 3. Image processing of CNT aggregates (a) Original image; (b) Removal of noise; (c) Thresholding; (d) Analyzed particle image. Thresholding; (d) Analyzed particle image.
In addition, particle number in the aggregated were counted from the original image (Figure In addition, particle number in the aggregated were counted the original (Figure of 3a) 3a) based on light spots on the aggregate because the catalysts for from the CNT synthesisimage is composed based on light spots on the aggregate because the catalysts for the CNT synthesis is composed of metal metal components with high reflectivity. components high reflectivity. Variouswith definitions can be used to analyze the shape of the aggregates. In this study, factors for Various the definitions can be used to were analyze the shape of on thethe aggregates. this et study, factors quantifying CNT aggregates shape obtained based studies of In Wang al. [1] and Hakim et al. [4], which incorporate various shape factors to visualize the aggregation phenomenon for quantifying the CNT aggregates shape were obtained based on the studies of Wang et al. [1] and of nanoparticles. Heywood diameter and Feret diameter were used for aggregatephenomenon diameter, andof Hakim et al. [4], which incorporate various shape factors to visualize thethe aggregation aspect ratio, roundness and solidity to quantify the for shape [15]. The Heywood diameter nanoparticles. Heywood diameter andwere Feretused diameter were used the aggregate diameter, and aspect refers to the diameter calculated by converting the particle area measured in two dimensions to theto ratio, roundness and solidity were used to quantify the shape [15]. The Heywood diameter refers area of the circle, and the Feret diameter, also called diameter, the distance between the diameter calculated by converting the particle areacaliper measured in twomeans dimensions to the area of the the two planes restricting the object to the a specific direction. roundness circle, andparallel the Feret diameter, also called caliperperpendicular diameter, means distance betweenThe the two parallel indicates the degree roundness of the edge the particle. TheThe closer to 1, the indicates closer to the planes restricting the of object perpendicular to a of specific direction. roundness the circle, degree the lower theedge value, the particle. longer the solidity theand degree of absence of ofand roundness of the of the Theellipse. closer toThe 1, the closerindicates to the circle, the lower the value, concave with respect to any shape, and the closer to 1, the convex surface only, which represents the the longer the ellipse. The solidity indicates the degree of absence of concave with respect to any shape, roughness of the [15]. surface only, which represents the roughness of the object [15]. and the closer to 1,object the convex
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3. Results The distributions of axial solid holdup of CNT particles at different gas velocity for CNT I and CNT II are shown in Figure 4. The gas velocity was increased from 0.09 m/s to 0.20 m/s in the bubbling fluidization regime. The initial bed height was 0.6 m, but the height of the dense phase was expanded to about 0.8 m as increasing of the gas velocity up to 0.20 m/s. This expansion of the dense bed is due to the unique structure of the Multi-walled CNTs (MWCNTs). Because the nanotubes on the CNTs have a long length and a narrow diameter, they become entangled easily during the growth of nanotubes on the particle surface. Then primary entangled MWCNTs between particles were formed like a chain-like structure [2]. The arrangement of CNT particles extends to the entire bed through the contact points between the networks of particles in the dense bed [4]. A dense bed with the entangled particle bed expands toward the top of the reactor as the gas velocity increases. The bed expansion is clearly observed in the CNT II with large mean size as in Figure 4b, indicating that the greater the particle size, the greater the entanglement between particles. The solid holdup (es ) is an indicator to show how many CNTs are distributed along the reactor height if the bed is in fluidization state by the Equation (1) [16]. es = ∆P/[∆L(ρp − ρg )g]
(1)
where ∆L is distance between two taps for the pressure drop measurement. The shape of the solid holdup distribution of the CNT particles showed an S-shape with a lower-dense phase, middle-transition region and an upper-dilute phase observed in a typical bubbling fluidized bed reactor. As the gas phase velocity increased, the solid holdup of the dense phase decreased. Also, the solid holdup in the transition region between the dense and dilute phases showed a tendency to increase due to the increase of drag on the CNT particles. In the dilute phase in freeboard region of the reactor, the solid holdup was also increased due to the increase of the particles entrainment by the increased gas velocity. The shape of the solid holdup distributions did not differ between the CNT I and CNT II particles. In the case of CNT I with small mean size, the solid holdup in the dilute phase was relatively higher than that of CNT II because the CNT I particles were easily entrained at a lower gas velocity. Meanwhile, the CNT II particles do not easily rise at the same velocity but stay at the bottom of the reactor, so the solid holdup in the dense bed was higher than the CNT I. And, the particles ejected from the bed surface are easily descended by the gravity, so that the solid holdup is relatively higher than that of CNT I in the transition region where the rising and falling of the particles coexist. Specifically, more pronounced particle scattering and aggregated particles were observed near the bed surface (0.7–1.0 m) in the CNT II experiment. The sizes and shape properties of the CNT aggregates were visualized and measured in the freeboard of the fluidized bed. Figure 5 shows the images of the aggregates and typical aggregates shapes on the light sheet in the dilute phase at different gas velocity. The bright spots represent particle aggregates. The aggregates have different size and shape depending on the gas velocity and the CNT particle type. The volume fraction of the aggregates was increased with increasing of gas velocity. The observed number of the CNT I aggregates was higher than that of the CNT II at the same gas velocity due to the higher entrainment of the smaller particles. Both the CNT I and CNT II were found to increase in the aggregates size as the gas velocity increased. Interestingly, the changes in aggregate shape with gas velocity were different for the two particles. At low gas velocities, it is observed that the aggregates of both CNTs were close to the circle and had smooth surface. However, the CNT II particles had a relatively non-uniform shape and more aggregates with several particles attached to each other compared to the CNT I particles above 0.13 m/s of gas velocity.
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Figure 4.4.Axial solid holdup distributions of CNTparticles particleswith with gas velocity CNT I; (b) CNT Figure Axial solid holdupdistributions distributions of gas velocity (a)(a) CNT I; (b) II. II.II. Figure 4. Axial solid holdup of CNT CNT particles with gas velocity (a) CNT I;CNT (b) CNT
Figure 5. Cont.
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Figure 5. Original images (180 mm wide × 80 mm high) and representative aggregates at given gas
Figure 5. Original images (180 mm wide × 80 mm high) and representative aggregates at given gas velocities. (A) CNT I, (B) CNT II. velocities. (A) CNT I, (B) CNT II.
Figure 6 shows the Feret diameter and the Heywood diameter variations of the CNT aggregates in the phase gas velocity. AsHeywood shown indiameter the figure, the meanofdiameter the Figuredilute 6 shows the with Feretthe diameter and the variations the CNTofaggregates aggregates increased as the gas velocity increased. In comparison between the CNTs, the aggregate in the dilute phase with the gas velocity. As shown in the figure, the mean diameter of the aggregates size of CNT II was larger than that of the CNT I, but both CNTs show a similar tendency in change of increased as the gas velocity increased. In comparison between the CNTs, the aggregate size of CNT II the aggregates size with gas velocity. This means that the gas velocity is an important factor in the was larger than that of the CNT I, but both CNTs show a similar tendency in change of the aggregates change of aggregate size. These results are consistent with the results of Kim [8], which reported that size with gas velocity. This means that the gas velocity is an important factor in the change of aggregate the aggregate diameter of multi-walled CNT particles in the dilute phase increased with gas velocity size.as These in theresults Figureare 6. consistent with the results of Kim [8], which reported that the aggregate diameter of multi-walled CNT in the dilute with gasgas velocity as depends in the Figure 6. The variation particles of the aggregates size phase in the increased dilute phase with velocity on the The variation ofthe thecontact aggregates in the diluteand phase with gasofvelocity on the aggregation due to betweensize the CNT particles the frequency the risingdepends and falling individual particles The between increase inthe theCNT aggregates sizeand withthe increasing gas velocity is primarily aggregation due to the[6,8]. contact particles frequency of the rising and falling due to particles the entrainment of increase large aggregates formed insize the with denseincreasing phase into the dilute is phase. individual [6,8]. The in the aggregates gas velocity primarily However, at the same time, the amount of entrained small CNT particles also increases, so that the due to the entrainment of large aggregates formed in the dense phase into the dilute phase. However, average diameter shows a minimum value around 0.11 m/s. However, the mean diameters increase at the same time, the amount of entrained small CNT particles also increases, so that the average for further increases of gas velocity, even as the entrainment rate of the smaller particles increases, diameter shows a minimum value around 0.11 m/s. However, the mean diameters increase for further which is due to the formation of the larger aggregates by the strong van der Waals forces between increases of gas velocity, even as the entrainment rate of the smaller particles increases, which is due the small CNT particles or between the small particles and the primary aggregates [17]. It is also to the formation larger aggregates the strong van derfrom Waals between the small CNT believed that of thethe entrained nanotubes, by which are separated theforces particles by inter-particle particles or between the bed, smallcontribute particlestoand primary aggregates also believed that the attrition in the dense thethe increase of the aggregates[17]. size Itbyisattaching to other entrained nanotubes, which are[18]. separated from the particles byininter-particle attrition the dense particles in the dilute phase This is better explained later Figure 9 on the particleinnumber in bed, the aggregates. contribute to the increase of the aggregates size by attaching to other particles in the dilute phase [18].
This is better explained later in Figure 9 on the particle number in the aggregates.
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Figure aggregates diameter. diameter. (a) (a) Feret Feret diameter; diameter; (b) (b) Heywood Heywood diameter. diameter. Figure 6. 6. Effect Effect of of gas gas velocity velocity on on aggregates aggregates diameter. (a) Feret diameter; (b) Heywood diameter.
The clarified by comparing The difference difference in in size size change change between between the the CNT CNT III and and CNT CNT II II can can be be clarified clarified by by comparing comparing the the The difference in size change between the CNT and CNT II can be the initial size distributions of the both CNTs and the changes in aggregates size distribution at each gas initial size size distributions distributions of of the the both both CNTs CNTs and and the the changes changes in in aggregates aggregates size size distribution distribution at at each each gas gas initial velocity as shown in Figure 7. ItIt is seen that the aggregates size changes not only in the average but velocity as shown in Figure 7. is seen that the aggregates size changes not only in the average but velocity as shown in Figure 7. It is seen that the aggregates size changes not only in the average but also in the distribution with the gas velocity. Despite the increase in the number of fine particles also in inthe thedistribution distributionwith with velocity. Despite the increase the number fine particles also thethe gasgas velocity. Despite the increase in theinnumber of fineofparticles rising rising into the dilute phase of the fluidized bed, the fraction of fine particles less than 200 μm was rising into the dilute phase of the fluidized bed, the fraction of fine particles less than 200 μm was into the dilute phase of the fluidized bed, the fraction of fine particles less than 200 µm was greatly greatly reduced in both CNTs compared to the initial particle size distribution. This indicates that greatly reduced in both CNTs compared to the initial particle size distribution. This indicates that reduced in both CNTs compared to the initial particle size distribution. This indicates that the fine the fine particles formed aggregates of larger size by mutual adhesion by the cohesive force. This the fine particles formed aggregates larger size byadhesion mutual adhesion by the force. cohesive This particles formed aggregates of larger of size by mutual by the cohesive Thisforce. becomes becomes clearer at aa high gas velocity of 0.17 m/s, where particles larger than the initial mean size of becomes clearer at high gas velocity of 0.17 m/s, where particles larger than the initial mean size of clearer at a high gas velocity of 0.17 m/s, where particles larger than the initial mean size of each each CNT are significantly increased. In comparison between the CNTs, the particle size distribution each CNT are significantly increased. In comparison between the CNTs, the particle size distribution CNT are significantly increased. In comparison between the CNTs, the particle size distribution of of CNT II shows aa larger change than CNT II, the II with size and larger of the the CNT shows larger change II, because because theICNT CNT with smaller smaller andsurface larger the CNT I shows a larger change than than CNT CNT II, because the CNT with smaller size andsize larger surface area has stronger cohesive force than the CNT II particles. In addition, the nanotubes surface area has stronger cohesive force than the CNT II particles. In addition, the nanotubes area has stronger cohesive force than the CNT II particles. In addition, the nanotubes separated from separated from the particles would have been aggregated into cohesive I, separated from thehave particles been further further aggregated into sufficiently sufficiently cohesive CNT CNT the particles would beenwould furtherhave aggregated into sufficiently cohesive CNT I, contributing to theI, contributing to the larger change in the size distribution of the CNT I. contributing change in the sizeCNT distribution of the CNT I. larger changeto inthe thelarger size distribution of the I.
Figure Figure 7. 7. Variation Variation of of CNT CNT particle/aggregates particle/aggregates diameter diameter distributions distributions with with gas gas velocity. velocity. (a) (a) CNT CNT I,I, Figure 7. Variation of CNT particle/aggregates diameter distributions with gas velocity. (a) CNT I, (b) CNT II. (b) CNT CNT II. II. (b)
Effect Effect of of gas gas velocity velocity on on shape shape factors factors of of the the CNT CNT aggregates aggregates in in the the dilute dilute phase phase is is shown shown in in Effect of gas velocity on shape factors of the CNT aggregates in the dilute phase is shown in Figure 8. The aspect ratio increases with increasing of gas velocity due to the increase of aggregation Figure 8. The aspect ratio increases with increasing of gas velocity due to the increase of aggregation Figure 8. the The aspect ratio increasesFigure with increasing of gas velocity due to the increase of aggregation between between the particles particles as as shown shown in in Figure 8a. 8a. The The aspect aspect ratio ratio of of CNT CNT II showed showed aa moderate moderate increase, increase, between the particles asa shown in Figure 8a. 0.13 The aspect ratio of CNTthe I showed acontributing moderate increase, but but the the CNT CNT IIII showed showed a sharp sharp increase increase after after 0.13 m/s, m/s, indicating indicating that that the factors factors contributing mainly mainly butthe theaggregation CNT II showed a sharp increase after 0.13 m/s, indicating that the factors contributing mainly to of CNT particles are different between the CNTs. As mentioned in Figure to the aggregation of CNT particles are different between the CNTs. As mentioned in Figure 7, 7, the the to theI aggregation of CNT particles are different between the CNTs. As mentioned in Figure 7, CNT aggregates are formed and grown mainly due to the aggregation of inter-particles CNT I aggregates are formed and grown mainly due to the aggregation of inter-particles and and the CNTnanotubes I aggregates aare formed and grown [18]. mainly due to the aggregation of inter-particles and particleparticle- nanotubes as as a secondary secondary contributor contributor [18]. The The CNT CNT IIII particles particles have have the the enhanced enhanced physical physical particlenanotubes as a secondary contributor [18]. The CNT II particles have the enhanced physical entanglement entanglement between between large large particles particles and and aggregates aggregates with with increasing increasing the the gas gas velocity, velocity, resulting resulting in in entanglement between large particles and aggregates with increasing thestudy gas velocity, resulting in the rapid increase of the aspect ratio. These results coincide well with the of Horio the rapid increase of the aspect ratio. These results coincide well with the study of Horio et et al. al. [11], [11], the rapid increase of the aspect ratio. These results coincide well with the study of Horio et al. [11], which which reported reported the the formation formation of of FCC FCC (Fluid (Fluid Catalytic Catalytic Cracking) Cracking) catalyst catalyst aggregates aggregates with with high high aspect aspect
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Processes 2018,dilute 6, x FOR PEER REVIEW 9 of 12to ratio in the phase of the circulating fluidized bed. However, there is a difference compared their study in that the increase of the aspect ratio in the CNT aggregates is due to physical factor ratio in the phase of the circulating fluidized bed.particles However, is a difference compared to such thedilute entanglement ofof the nanotubes on the inthere addition to thewith inter-particular whichas reported the formation FCC (Fluid Catalytic Cracking) catalyst aggregates high aspect their study in that the increase of the aspect ratio in the CNT aggregates is due to physical factor cohesive force. Figure 8b,c the variations in roundness and solidity aggregatescompared with gas ratio in the dilute phase ofshow the circulating fluidized bed. However, thereofisthe a difference such as the entanglement of the nanotubes on the particles in addition to the inter-particular velocity. As the gas velocity increased, both roundness and solidity decreased, and the decrease was to their study in that the increase of the aspect ratio in the CNT aggregates is due to physical factor cohesivethe force. Figure 8b,c show the variations in roundness and is solidity of the aggregates with gas larger CNT II. The rapid of the CNT II roundness duetotothe the physical entanglement such asinthe entanglement of thedecrease nanotubes on the particles in addition inter-particular cohesive velocity. As theparticles, gas velocity increased, bothan roundness andwith solidity decreased, and theas decrease was of each CNT thereby forming aggregate non-uniform shape shown and force. Figure 8b,c show the variations in roundness and solidity of the aggregates with gas velocity. larger in the CNT II. The rapid decrease of the CNT II roundness is due to the physical entanglement discussed in Figure 5 and Figure 8a. On the other hand, a gradual decrease in the roundness of the As the gas velocity increased, both roundness and solidity decreased, and the decrease was larger of each CNT particles, thereby forming an aggregate with non-uniform shape as shown and CNT particles is because the fine of particles andIIthe separated evenly adhered to the in theI CNT II. The rapid decrease the CNT roundness is nanotubes due to the are physical entanglement of discussed in Figure 5 and Figure 8a. On the other hand, a gradual decrease in This the roundness of the surface of the particles or aggregates during the inter-particle aggregation. also results in a each CNT particles, thereby forming an aggregate with non-uniform shape as shown and discussed CNT I particles is because the fine particles and the separated nanotubes are evenly adhered to the gradual reduction in the solidity by reducing the formation of concavities that can occur during the in Figures 5 and 8a. On the other hand, a gradual decrease in the roundness of the CNT I particles surface of the particles or aggregates during the inter-particle aggregation. This also results in a aggregate is becauseformation. the fine particles and the separated nanotubes are evenly adhered to the surface of the gradual reduction in the solidity by reducing the formation of concavities that can occur during the particles orformation. aggregates during the inter-particle aggregation. This also results in a gradual reduction in aggregate the solidity by reducing the formation of concavities that can occur during the aggregate formation.
Figure 8. Effect of gas velocity on aggregates shape factor. (a) aspect ratio, (b) roundness, (c) solidity. Figure 8. Effect of gas velocity on aggregates shape factor. (a) aspect ratio, (b) roundness, (c) solidity.
Figure 8. Effect of gas velocityon onthe aggregates factor. (a) particles aspect ratio, (c) is solidity. The effect of gas velocity numbershape of the CNT in (b) theroundness, aggregates shown in FigureThe 9. Since the CNT particles used in this study are prepared by growing carbon nanotubes on effect of gas velocity on the number of the CNT particles in the aggregates is shown in The effect of gas velocity on the number of the CNT particles in the aggregates is shown in Figure 9. the iron9.catalysts [19], theparticles iron catalysts highare reflectivity strong bright when on the Figure Since the CNT used inhaving this study preparedshow by growing carbonspots nanotubes Since theare CNT particleswith usedlaser in this study are prepared by growing carbon nanotubes on the iron particles irradiated light. Therefore, the number of CNT particles contributing the the iron catalysts [19], the iron catalysts having high reflectivity show strong bright spots whentothe catalysts [19], the iron catalysts having high reflectivity show strong bright spots when the particles formation of aggregates can be determined by measuring the number of catalysts. In the particles are irradiated with laser light. Therefore, the number of CNT particles contributing to the are irradiated with laser light. Therefore, the number of CNT particles contributing to the formation of experimental velocity range, number ofby CNT particles found in the of aggregates formation of gas aggregates can bethe determined measuring the number catalysts.increased In the aggregates be determined by measuring theofnumber ofofcatalysts. In the experimental gas velocity from 1.1 to can 1.5–1.8 per aggregate, and the increasing rate the number relatively higher at the experimental gas velocity range, the number CNT particles found inwas the aggregates increased range, the number of CNT particles found in the aggregates increased from 1.1 to 1.5–1.8 per aggregate, CNT with large surface area due to the theincreasing strong cohesion at the samewas gas relatively velocity. However, the from I1.1 to 1.5–1.8 per aggregate, and rate of the number higher at the and the increasing rate of the number was relatively higher at the CNT I with large surface area particle number in the aggregates is relatively small, and cannot be fully explained by inter-particle CNT I with large surface area due to the strong cohesion at the same gas velocity. However, the due to the strong cohesion at the same gas velocity. thebeparticle number in aggregates aggregation as anin only cause of aggregate formation, considering the result in Figure 7 on the size particle number the aggregates is relatively small,However, and cannot fully explained by the inter-particle is relatively small, and cannot be fully explained by inter-particle aggregation as an only cause change of aggregate. aggregation as an only cause of aggregate formation, considering the result in Figure 7 on the sizeof aggregate change of formation, aggregate. considering the result in Figure 7 on the size change of aggregate.
Figure Figure 9. 9. Effect Effect of of gas gas velocity velocity on on number number of of CNT CNT particles particles in in aggregates. aggregates. Figure 9. Effect of gas velocity on number of CNT particles in aggregates.
It is once again proven that the separated nanotubes contribute significantly to the formation of It is once again proven that the separated nanotubes contribute significantly to the formation It is once again proven the [17,18]. separated contribute to the formation of the aggregates in small sizethat CNTs Fornanotubes the confirmation ofsignificantly the above description, the fine of the aggregates in small size CNTs [17,18]. For the confirmation of the above description, the fine the aggregates in small size CNTs [17,18]. For the confirmation of the above description, the fine particles in the filter connected to the cyclone exit were recovered after the completion of particles in the filter connected to the cyclone exit were recovered after the completion of experiments, particles in and the it filter theof cyclone exit less were recovered afterseparated the completion of experiments, was connected found that to most the particles than 10 μm were nanotubes. and it was found that most of the particles less than 10 µm were separated nanotubes. experiments, and it was found that most of the particles less than 10 μm were separated nanotubes.
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From the present results, it can be seen that the CNT particles and the primary aggregates entrained from the dense phase are aggregated into larger aggregates with various shapes, and the aggregates dominatethe thehydrodynamic hydrodynamic behavior in dilute the dilute of the fluidized The aggregates dominate behavior in the phasephase of the fluidized bed. Thebed. physical physical properties of aggregates in the dilute are very for important forflow gas-solid flow properties of aggregates in the dilute phase are phase very important gas-solid simulation, simulation, cyclone and heatmodeling transfer modeling whichaverage require average and shape cyclone design anddesign heat transfer which require particle particle size andsize shape factor factor of particles [12,13,20]. Therea were a few correlations predicting the aggregate size of particles the FCC of particles [12,13,20]. There were few correlations predicting the aggregate size of the FCC particles in the dilute phase [11,21], but theoncorrelation on the has CNT particles has been In rarely in the dilute phase [11,21], but the correlation the CNT particles been rarely reported. this reported. In this study, firstly, a correlation size of the aggregates study, firstly, a correlation for predicting the for sizepredicting of the CNTthe aggregates in CNT the dilute phase in of the dilute phase of the fluidized bed has been proposed. obtained Heywood diameters the fluidized bed has been proposed. The obtained Heywood The diameters of the aggregates in dilute of phase aggregates in dilute phasethis of study the fluidized bed from this study literature in Table 1 have been of the fluidized bed from and literature in Table 1 haveand been correlated with experimental correlated with experimental parameters as following. parameters as following. (dH /dp ) = 4.43Ar−0.42 Re1.56 Fr −1.20 (2) (dH/dp) = 4.43Ar−0.42Re1.56Frpp−1.20 (2) The correlation correlation coefficient coefficient of A good good agreement agreement The of Equation Equation (2) (2) is is 0.98 0.98 and and standard standard error error is is 0.100. 0.100. A between the between the measured measured and and calculated calculated values values from from the the Equation Equation (2) (2) was was shown shown as as in in Figure Figure 10. 10. Table 1. A summary of experimental conditions of the present and previous studies. Table 1. A summary of experimental conditions of the present and previous studies. 3) 3) Particles dp (μm) dp (µm)ρp (kg/m ρp (kg/m Particles CNT I I 131 131 229 229 CNT study ThisThis study CNT CNT II II 220 220 229 229 KimKim [8] [8] CNT 291 291 188 188 CNT
Ar (-) Ar (-) 17.77 17.77 84.18 84.18 159.76 159.76
Re(-) (-) Re 2.48–4.92 2.48–4.92 2.48–5.93 2.48–5.93 1.37–2.04 1.37–2.04
FrpFr (-)p (-)
1.84–4.40 1.84–4.40 2.39–4.72 2.39–4.72 1.08–1.60 1.08–1.60
Figure 10. Comparison of the calculated Heywood diameter of aggregates and the experimental data. Figure 10. Comparison of the calculated Heywood diameter of aggregates and the experimental data.
4. Conclusions
4. Conclusions The effect of the CNT particle size on the aggregates properties has been determined. The CNT aggregates size,ofparticle number in size the aggregates and aspect ratio increased the roundness and The effect the CNT particle on the aggregates properties has beenbut, determined. The CNT the soliditysize, decreased with increasing velocity.and Two CNTratio particles of different mean size aggregates particle number in the gas aggregates aspect increased but, theinitial roundness and showed different tendency in variations of the aggregates size and shape factors with gas velocity due the solidity decreased with increasing gas velocity. Two CNT particles of different initial mean size to differences in factors contributing to the of aggregate formation such physical entanglement showed different tendency in variations the aggregates size andasshape factors with gas between velocity large CNT particles and cohesive force between fine particles and separated nanotubes. The CNT due to differences in factors contributing to the aggregate formation such as physical entanglement particles with the larger mean size presented as relatively larger in the aggregate size than the smaller between large CNT particles and cohesive force between fine particles and separated nanotubes. The CNT particles particles with at given velocities. The aggregates from the large particles showed a sharp CNT the gas larger mean size presented as relatively largerCNT in the aggregate size than the smaller CNT particles at given gas velocities. The aggregates from the large CNT particles showed a sharp increase in the aspect ratio and rapid decrease in the roundness and the solidity with gas
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increase in the aspect ratio and rapid decrease in the roundness and the solidity with gas velocity. The obtained diameters of aggregates have been firstly correlated with the experimental parameter. Funding: This research was funded by the National Research Foundation of Korea (NRF). Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A3B03029917). Conflicts of Interest: The authors declare no conflict of interest.
Notifications Archimedes number (dp 3 ρg (ρp − ρg ) g/µ2 ) (-) reactor diameter (m) Heywood diameter (m) mean particle diameter (m) particle Froude number (Ug /(g dp )0.5 ) (-) gravitational constant (m/s2 ) distance between two taps (m) Reynolds number (ρg DUg /µ) (-) Pressure drop (pa) gas velocity (m/s)
Ar D dH dp Frp g ∆L Re ∆P Ug Greek εs µ ρg ρp
solid holdup (-) gas viscosity (kg/m s) gas density (kg/m3 ) apparent particle density (kg/m3 )
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